There follows, with reference to FIGS. 1 to 8, a general description of lines for applying treatment to strips of steel or aluminum.
A vertical cooling chamber of a line for applying treatment of strips of steel or aluminum made in accordance with the prior art is constructed on the principle shown in FIG. 1, in which there can be seen a cooling chamber 4 of a treatment oven, through which there passes a strip 1 of steel or aluminum, which strip is subjected to the action of cooling elements 2 on passing over top deflection rollers 3 and bottom deflection rollers 3′. The strip 1 is cooled in the chamber 4 mainly by the cooling elements 2 that are constituted by assemblies for blowing gas at a temperature lower than the temperature of the strip.
On passing through the cooling chamber 4, the strip 1 is cooled on both faces by the cooling elements 2 situated on either side of the line of travel, and when cooling is performed on a plurality of lines of travel, said strip changes its line of travel each time it goes past a deflection roller 3 or 3′. The strip cooling curve in the chamber is controlled by indexing the various cooling elements 2 or groups of cooling elements operating in identical manner.
A vertical cooling section of a line of travel for strips of steel or aluminum made in accordance with the prior art is constructed on the principle shown in FIG. 2, in which there can be seen a vertical cooling section 10 through which there passes a strip 11 that is subjected to the action of cooling elements 12. The strip 11 is cooled within the section mainly by the cooling elements 12 that are constituted by assemblies for blowing air at a temperature lower than the temperature of the strip. The ideal line of travel for the strip 11 is determined by the top deflection roller 13 and the bottom deflection roller 13′.
On passing through the cooling section 10, the strip 11 is cooled on both faces by the cooling elements 12 situated on either side of the line of travel. The cooling curve for the strip in the section is controlled by indexing the various cooling elements 12 or groups of cooling elements that operate in identical manner.
The productivity of the cooling section or chamber is determined by the capacity for transferring heat in cooling so as to ensure that the strip at the outlet of the cooling section or chamber achieves temperatures and the cooling rates (expressed in ° C./second) that are suitable for determining the metallurgical quality of the finished product. The heat transfer depends on the blow distance between the strip and the cooling system, on the geometrical configuration of the blowing, and on the blow speed. Heat transfer is also made more effective if the blow distance is short and/or if the blow speed is large.
Increasing the blow speed and decreasing the distance between the strip and the blow system beyond a certain limit leads to vibration and/or oscillation of the strip that can lead to the strip coming into contact with the blow system (or with means for protecting the blow system), thereby leading to marking (scratching) that is incompatible with the desired surface quality, and even in extreme circumstances leading to the strip breaking.
Increasing the performance of lines for treating steel or aluminum requires faster cooling rates on products that are becoming finer and finer and wider and wider.
For example, when annealing strips of steel, it is not uncommon to specify, for the cooling chamber of a continuous annealing oven, cooling rate requirements that are severe (typically greater than 80° C./second) for steels said to be of drawing quality (DQ), deep drawing quality (DDQ), and high strength steel (HSS). Cooling rates are slower (typically 20° C./second) for so-called commercial quality (CQ) steels. Document EP 0 803 583 A2 describes this need and the various applications.
It should be observed that the proportion of steels having a high stamping limit (e.g. of the DDQ type) or having a high elastic limit (e.g. of the HSS type) is increasing significantly.
Likewise, in order to save weight, in particular in automobile applications, the mean thickness of steels is decreasing, whereas the mean width of the sheets for treatment is increasing with optimization of the stamping means.
Finally, the capacities of treatment lines, and in particular of annealing or galvanization lines is increasing towards ever greater capacities.
This increase, combined with the various parameters mentioned above, is leading to a new problem appearing in cooling sections or chambers, namely strip vibration, where this phenomenon used to be limited or even unknown in equipment made in accordance with the prior art.
Naturally, this phenomenon is very critical for vertical sections or chambers as shown in FIGS. 1 and 2, but it also exists with a horizontal line of travel, even though the phenomenon is then attenuated by the weight of the strip.
The post-coating cooling zone in a hot galvanization line as shown in FIG. 3 is also very sensitive to this phenomenon. After a steel strip 21 has been coated by being immersed in a bath 22 of molten zinc alloy, the thickness of the coating is controlled by wiping the liquid coating in air or nitrogen. This wiping is generally performed by a pair of blow nozzles 23, 23′. The following vertical cooling zone 24 is for the purpose of solidifying the coating and ensuring that, on reaching the deflection roller 25 at the top of the tower, the strip is at a temperature that is compatible with the process, in particular that avoids leaving any traces on the coating.
On large-capacity lines, increasing capacity means that the height of the free strand of strip 21 between the last roller 26 immersed in the molten zinc bath 22 and the high deflector roller 25 in the tower exceed 50 meters (m).
It is desirable to reduce this height both for technical and for economic reasons, but that would lead to increasing heat exchange coefficients, and once again that would generate levels of vibration that are incompatible with the quality of the finished product. Such vibration can lead to marking by the strip coming into contact with external elements, and it is also harmful to the regularity of the zinc coating. One of the essential parameters of wiping is distance between the blow nozzle 23 or 23′ and the strip 21, following a line of travel that, ideally, is unchanging. The vibration of the strip 21 leads to a change in the line of travel in the longitudinal and/or transverse direction of the strip, and thus to coating that is not uniform.
In order to limit the undesirable effects of strip vibration, attempts have been made in a prior technique to limit vibration by reducing the length of the blow boxes (or zones), so as to be able to install stabilizing rollers. Nevertheless, that technique limits the length subjected to cooling and thus limits the effectiveness of the cooling in the zone, and furthermore that technique requires the strip to come into contact with the stabilizer rollers, which is incompatible with applications in cooling zones following hot galvanization since the coating is not totally solidified.
Air-flow stabilization systems have also been proposed for replacing the above-mentioned stabilizer rollers. Those systems are relatively effective and they can contribute to cooling, however they are not optimized for enhancing the heat exchange coefficient, and thus for optimizing cooling. In addition, energy consumption is relatively large.
Another attempt has consisted in increasing traction on the strip, however that solution can be envisaged only for strips that are of considerable thickness, and for strip temperatures that are low, since the thermomechanical stresses generated on fine strips at high temperature can exceed the elastic limit of the strips and can lead to deformation that is permanent, or even to the strip breaking.
Another solution consists in controlling vibration of the strip by adapting the blow speed, and/or the distance between the strip and the blow elements, and/or the blow rate in the event of vibration appearing. That leads to limiting the effectiveness of cooling, and thus to limiting the performance of the installation.
As shown in FIG. 4, another solution has been proposed for encouraging the blown gas to flow laterally. That solution consists in arranging blow tubes 31, 31′ on blow boxes 32, 32′ situated on either side of the strip 33 which travels in the direction referenced 100. The blow tubes 31, 31′ can thus guide the blow jets 34, 34′ that are emitted in a direction that is perpendicular to the plane of the traveling strip 33. Although that system leads to an improvement compared with boxes that merely have holes, it constitutes a solution that is not satisfactory, and the strip is observed to wander in such systems, thus leading either to damage to the tubes when the strip is thick, or to the strip breaking when the strip is fine. Since the gas, once blown, can be exhausted only towards the sides of the boxes, either in the travel direction of the strip or else laterally, it follows that a large flow of gas travels parallel to the strip in a volume that is confined between the strip and the boxes on its way to the edges of said boxes. The presence of the tubes 31, 31′ does in fact increase the volume available that is confined between the strip and the boxes, compared with boxes that merely have holes.
The disturbances that have been observed with the arrangement of FIG. 4 are illustrated in FIGS. 5 and 6 which are end views seen looking along arrow A in FIG. 4.
In FIG. 5, simulations involving fluid mechanics as applied to industrial configurations demonstrate that when the strip 33 is off-center towards one or other of the boxes, in this case the box 32′, the resultant of the pressures acting on the strip exerts a force F tending to move the strip even closer to said box. The system is thus unstable and does not tend to stabilize the strip on a line of travel that is centered between the boxes. In FIG. 6, simulations of fluid mechanics on industrial configurations show that when the strip 33 is inclined, the resultant of the pressures exerted on the strip exerts torque T tending to incline the strip even more, and thus to move the edges of the strip towards the boxes. The system is thus likewise unstable, and there is no tendency for the strip to be stabilized on a line of travel that is centered between the boxes. The results of FIGS. 5 and 6 have been demonstrated by software for simulating fluid mechanics, and by calculating the resultant of the pressures exerted on each face of the strip. The resultant of the pressures exerted on each face of the strip is the resultant of positive pressures in zones that are substantially in register with the blow tubes and of negative pressures in register with portions that are not situated in register with said tubes.
As described in document WO-A-01/09397, proposals have been made to channel the flow of blown gas by providing for the blow tubes to be inclined towards the edges of the strip, mainly for the purpose of improving cooling, however modeling reveals only a small improvement in the effects shown diagrammatically in FIGS. 5 and 6.
U.S. Pat. No. 6,054,095 also teaches inclining the blow tubes of the boxes towards the edges of the strip, but for the purpose of obtaining better treatment uniformity in the strip, and thus without being concerned about the stability of said strip while it travels. In a variant, U.S. Pat. No. 4,673,447 describes the use of blow boxes having holes, said holes being arranged through a plate that is thick so as to impart inclinations to the jets of gas. It should be observed that the jets are inclined not towards the edges, but on the contrary towards a midplane, symmetrically about said plane. That thus constitutes more of a mere stabilizing skid.
Document EP-A-1 108 795 describes a variant of the above techniques in which boxes are used that have straight blow tubes (perpendicular to the plane of the strip). The idea is merely to modify the intensity of cooling by acting on the lengths of the tubes, which tubes are selected to be shorter near the edges of the strip.
Document EP-A-1 029 933 describes another variant with boxes having nozzles in the form of blades. The transverse blades do not produce any inclined jets, and the boxes do not make it possible to organize recovery of the blow gas perpendicularly to the strip, as already mentioned above.
In another design, and in order to limit the flow of gas in a direction parallel to the travel direction of the strip, a solution in widespread use is as shown in FIGS. 7 and 8 (FIG. 8 being a section on VIII-VIII of FIG. 7). That solution consists in using tubular blow nozzles 41 each having an axis 48, two end walls 46, and a gas inlet 47, said nozzles being pierced by respective pluralities of circular holes 42 that are oblong or slot-shaped, enabling jets 45 to be blown against the strip 43 traveling in the direction 100. Even if the confinement between the strip 43 and the blow nozzles 41 is smaller than in arrangements making use of boxes having tubes, and does enable a certain amount of gas to be recovered in a direction normal to the plane of the strip between the blow nozzles, that confinement leads to pressure effects that are most unfavorable, leading to the same phenomena as those described with reference to FIGS. 5 and 6. That result can be demonstrated by modeling the suctions created by that configuration, and the strip is not stabilized on an optimum line of travel, i.e. a line centered between the blow nozzles.
Finally, document EP 1 067 204 A1 describes a solution for suppressing vibration by adjusting the pressure and/or the flow rate of gas that is blown transversely relative to the strip. In addition to the complexity of the adjustment that needs to be adapted to each product for treatment, that method presents two major drawbacks. Firstly, the strip can be caused to depart from being parallel with the blow devices, thereby reducing the distance between the strip and the device and increasing the risk of contact. Finally, cooling capacity is not maximized, and the reduction in the speed and/or pressure against one face cannot be compensated by an increase in the speed or the pressure of the jets against the other face if the limits on speed or on blow capacity have already been reached.